Journal of Alloys and Compounds 653 (2015) 228e233
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Efﬁcacy of In2S3 interfacial recombination barrier layer in PbS quantum-dot-sensitized solar cells Muhammad Abdul Basit a, 1, Muhammad Awais Abbas b, 1, Jin Ho Bang b, c, d, **, Tae Joo Park a, b, * a
Department of Materials Science & Engineering, Hanyang University, Ansan, 15588, Republic of Korea Department of Advanced Materials Engineering, Hanyang University, Ansan, 15588, Republic of Korea Department of Chemistry and Applied Chemistry, Hanyang University, Ansan, 15588, Republic of Korea d Department of Bionanotechnology, Hanyang University, Ansan, 15588, Republic of Korea b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 July 2015 Received in revised form 14 August 2015 Accepted 28 August 2015 Available online 1 September 2015
In2S3 interfacial recombination barrier layer (IBL) via successive ionic layer adsorption and reaction (SILAR) was successfully employed between PbS quantum dots and mesoporous TiO2 in quantum-dotsensitized solar cells (QDSSCs). In2S3 IBL signiﬁcantly increased the resistance against back electron transfer from TiO2, resulting an increment in the photocurrent density (JSC) for the cell with single SILAR cycle of In2S3 IBL. Further increase in the number of SILAR cycles of In2S3 IBL deteriorated the JSC, whereas open-circuit voltage sustained the increasing trend. Therefore, an optimal photo-conversion efﬁciency of ~2.2% was obtained for the cell with 2 SILAR cycles of In2S3 IBL, which strategically reached a value of ~2.70% after annealing (increased by 40% compared to the control cell without IBL). In2S3 IBL not only improved the recombination resistance and electron life time of the cells, but it also enhanced the photostability of the cells. © 2015 Elsevier B.V. All rights reserved.
Keywords: QDSSCs Interfacial recombination barrier Indium sulﬁde SILAR
1. Introduction Among third generation solar cells, quantum-dot-sensitized solar cells (QDSSCs) have gained prominent status owing to their unique characteristics like easy fabrication, cost effectiveness and competence to exhibit improved photovoltaic performance [1e3]. Despite of the fact that the maximum conversion efﬁciency of QDSSCs is still less than that of dye-sensitized solar cells (DSSCs) , the energy band tunability  and tendency to incorporate multiple sensitizers  concurrently have made QDs a suitable alternative to expensive dyes for solar cell application. Unlike commonly used dyes in DSSCs, QDs offer pragmatic light absorption in the near infrared range [7e9]. This extended light absorption in QDSSCs is advantageous to make use of unutilized portion of incident light for charge carrier generation, ensuing superior
* Corresponding author. Department of Materials Science & Engineering, Hanyang University, Ansan, 15588, Korea. ** Corresponding author. Department of Advanced Materials Engineering, Hanyang University, Ansan, 15588, Korea. E-mail addresses: [email protected]
(J.H. Bang), [email protected]
(T.J. Park). 1 Both authors contributed equally to this work. http://dx.doi.org/10.1016/j.jallcom.2015.08.237 0925-8388/© 2015 Elsevier B.V. All rights reserved.
photocurrent density than DSSCs, in general. Metal chalcogenidetype QDs synthesized through solution chemistry methods such as chemical bath deposition (CBD)  and successive ionic layer adsorption and reaction (SILAR)  comprise a signiﬁcant area of exploration for extended spectral response in solar cells. PbS , PbSe , CdS , CdSe , as well as CuInS2  and AgInS2  are respectively binary and ternary metal chalcogenide-type materials which exhibited attractive photo-conversion efﬁciency (PCE) in past few years. In general, QDSSCs exceeding 4% photo-conversion efﬁciency have been considered as high performance solar cells [18e25]. Lee et al. and Santra et al. have employed doping strategy to achieve signiﬁcantly improved efﬁciency over 5% for QDSSCs based on PbS and CdS quantum dots, respectively [3,5]. However, Zhao and coworkers have very recently demonstrated a double layer treatment of QDs-sensitized photoanode to realize a record efﬁciency beyond 8% . Even though QDSSCs are theoretically proﬁcient enough to have a PCE of ~44% , various drawbacks such as interfacial recombination and back electron transfer restrict their performance [6,8,12]. In order to minimize the detrimental effect of back electron transfer, various techniques like atomic layer deposition (ALD), CBD, sol-gel and layer-by-layer (LbL) chemical method have
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been employed for depositing interfacial recombination barrier layer (IBL) in DSSCs [28e32]. However, there are few reports on such IBLs for carrier recombination control in QDSSCs. Therefore, in this study, we elucidated the efﬁcacy of SILAR deposited In2S3 IBLs for PbS-QDSSCs considering its nonhazardousness, easy ﬁlm deposition route and the higher bandgap energy (Eg, ~2.5 eV) [33,34] than PbS (~1.0 eV)  QDs. Incident photon-to-current conversion efﬁciency (IPCE) measurement, dark current density-voltage (J-V) measurement, open-circuit voltage decay (OCVD) technique and electrochemical impedance spectroscopy (EIS) were employed to reveal the effectual role of In2S3 IBL. 2. Experimental Transparent conducting glass (i.e., F-doped SnO2, FTO) substrates were cleaned with aqueous solution of HCl and then rinsed in acetone, ethanol and de-ionized water (DIW) for 30 min each. For all cleaning processes, ultrasonic bath was used. Screen printing method was used to spread nearly 15 mm-thick TiO2 ﬁlm on FTO. For ﬁrst three screen printing cycles, commercial TiO2 paste (Solaronix Ti-nanoxide T/SP) was used, whereas last two cycles of screen printing were done using scattering particles paste (Dyesol Timo TiO2 Paste 18 NR-AO). After each screen printing cycle, samples were heat treated at 125 C for 6 min for drying. Finally, all samples were sintered at 550 C for an hour to obtain mesoporous structure by removing the organic components of TiO2 commercial pastes. PbS-QDs sensitization of TiO2 photoanodes was done using SILAR process. Screen-printed TiO2 on FTO substrates was ﬁrst dipped for 60 s in 0.02 M methanol based Pb(NO3)2 solution, and then dipped in 0.02 M sulfur ion precursor. SILAR process was repeated for three times to obtain optimum PbS-QDs sensitization. In-between every dipping cycle, every sample was thoroughly rinsed in methanol to remove loosely adsorbed ionic species. For the deposition of In2S3 interfacial recombination barrier layer (IBL), 0.03 M solution of In(NO3)3 in methanol was used as indium ion precursor and 0.03 M Na2S solution as sulfur ion precursor. 1 to 3 SILAR cycles of In2S3 deposition on TiO2 photoanode were introduced to study the photovoltaic effect on the optimized PbSQDSSCs, while the dipping time was kept 60 s for both ionic precursors. All samples were lastly passivated by 2 cycles of ZnS deposition using SILAR process in which 0.05 M Zn(CH3COO)2 and 0.05 M sulfur ion precursor were used. All sulfur ion precursors were prepared by dissolving Na2S in water ﬁrst and then in methanol (1:1) . Surlyn ﬁlm was placed around QDs sensitized TiO2 ﬁlm and heated at 90 C for 5 min. After placing surlyn separator, polysulﬁde electrolyte composed of 0.25 M S/0.625 M Na2S in DIW and methanol (3:2) was dropped carefully on the QDs sensitized photoanode. CuS counter electrode prepared by CBD  was held together with photoanode using two steel clips. For the counter electrode, CuS ﬁlm was deposited on cleaned FTO glass substrates using a mixed aqueous solution of 1 M Na2S2O3 and 1 M CuSO4. Soon after mixing the two solutions in a 4:1 volumetric ratio, acetic acid was added to bring down the pH value to 2. FTO glass substrates were kept in the resultant solution at 70 C for 3 h, which were cleaned with DIW and annealed at 130 C for 30 min. The characteristic J-V curves were measured by Keithley 2400 which operated at an adjusted illumination of 1 sun (AM1.5 G). The light intensity of a solar simulator (HAL-320, Asahi Spectra) was standardized for J-V as well as OCVD measurements using a CS-20 silicon diode (Asahi Spectra). Morphological data acquisition along with elemental mapping was done using MIRA3 TESCAN highresolution scanning electron microscope (SEM) connected to an energy-dispersive spectrometer (EDS). A customized IPCE
equipment (QEX7, PV Measurements, Inc.) was used to carry out the incident photon to current conversion efﬁciency measurements in 400e1100 nm range of wavelength. A potentiostat (IM6ZAHNER) coupled with frequency response analyzer (Thales) was used to carry out EIS measurements, which were done at an AC amplitude of 10 mV under dark condition. 3. Results and discussion Fig. 1 shows SEM images of mesoporous TiO2, PbS-QDs sensitized TiO2 with and without In2S3 IBL (2 SILAR cycles) along with EDS statistics. Elemental analysis of these three samples through EDS (Fig. 1def) not only conﬁrmed the explicit presence of In in the sensitized photoanode (2.41% by weight), but it also revealed the enhanced concentration of Pb (from 8.75% to 12.8% by weight) in the TiO2 photoanode with In2S3 IBL, which is beneﬁcial to generate more carriers in the cells. Topographical SEM images at higher magniﬁcations are also included in Fig. S1. The enhanced loading of PbS on mesoporous TiO2 with In2S3 IBL was also supported by the EDS elemental mapping (Fig. S2). Similar effect of IBL on CdSe and CuInS2 QDs deposition has been reported previously [37,38]. The characteristic J-V curves for PbS-QDSSCs with and without In2S3 IBLs are shown in Fig. 2a, based on which Fig. 2bed summarize the photo current-density (JSC), open-circuit voltage (VOC), ﬁll factor (FF) and photo-conversion efﬁciency (h), respectively. Mathematically, efﬁciency of a solar cell is calculated from following Eq. (1),
ðJsc Voc FFÞ Pin
where Pin represents the intensity of incident light. As shown in Fig. 2b, the cell having In2S3 IBL with single SILAR cycle (1In2S3) exhibited an increase of ~20% (from ~12.9 to ~15.5 mA/cm2) in the JSC as compared to the control cell without In2S3 IBL. This is because the In2S3 IBL with higher conduction band-offset to TiO2 than PbS suppresses the undesirable carrier recombination through back-transfer of electrons from the conduction band of TiO2 to PbS-QDs or/and the electrolyte [39,40] (red dotted lines in web version) as shown in Fig. 2f. However, an increase in number of SILAR cycles (thickness) of In2S3 IBL reduced the JSC to 14.2 and 11.0 mA/cm2 at 2 and 3 SILAR cycles of In2S3 IBL (2In2S3 and 3In2S3), respectively, because the In2S3 IBL became too thick to sustain the electrons tunneling from PbS to TiO2 photoanode through IBL [28,41]. Such a decrease in JSC value of QDSSC with 3In2S3 IBL lowered down the efﬁciency to ~2%. The trend in JSC value was also afﬁrmed from IPCE spectra of the cell without and with the In2S3 IBLs (Fig. S3). The cell with 1In2S3 IBL showed the highest IPCE, which decreased with increasing thickness of In2S3 IBL. This is consistent with JSC trend in J-V curves. The resistance against back electron transfer increased simultaneously with the thickness of In2S3 IBL , which increased the VOC as shown in Fig. 2c. Persistent increase in the VOC and continuous decrease in the JSC results an improving trend in the FF of devices with increasing thickness of In2S3 IBL as shown in Fig. 2d. The consequent inﬂuence of stated variations in JSC, VOC and FF of the solar cells on PCE is reﬂected in Fig. 2e, which asserts that 2In2S3 IBL provides an optimum enhancement of ~18% in PCE (from 1.91 to 2.25%) by uplifting JSC from 12.9 to 14.19 mA/cm2 and VOC from 0.41 to 0.44 V. The charge carrier recombination characteristics of fabricated cells were examined to substantiate the anticipated efﬁcacy of In2S3 IBL using dark J-V measurement as shown in Fig. 3a, which is directly indicative of the resistance against electron back-transfer. The JSC under dark decreased with increasing thickness of In2S3
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Fig. 1. SEM images and EDS results for (a, d) mesoporous TiO2, (b, e) PbS-QDs sensitized TiO2 and (c, f) In2S3 IBL incorporated PbS-QDs sensitized TiO2 photoanode, respectively.
Fig. 2. (a) J-V characteristics of PbS-QDSSCs fabricated with and without In2S3 IBLs, (b) JSC, (c) VOC, (d) FF and (e) PCE as a function of number of SILAR cycles of In2S3 IBL. (f) Band diagram for mechanism of charge transfer and carrier recombination in PbS-QDSSCs with In2S3 IBL.
IBL, which necessarily happened due to reduction in recombination current. Furthermore, the increased VOC of the cells with thickness of In2S3 IBL in Fig. 2c can be also explained with reduced JSC in dark J-V curves based on Eq. (2)
nkT ln q
JL þ1 JO
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Fig. 3. (a) Dark J-V characteristics and (b) OCVD of PbS-QDSSCs with and without In2S3 IBLs.
where, JO and JL represent dark saturation current-density and light generated current-density, respectively. To endorse the charge transfer dynamics of PbS-QDSSCs with In2S3 IBL further, we carried out OCVD test (Fig. 3b), which is one of strong tools to examine the recombination behavior of QDSSCs. Eq. (3) narrates that the electron life time tn is inversely related to the 1 oc rate of OCVD dV . dt
k T tn ¼ B e
Simply, faster OCVD reﬂects lower electron life time and inferior recombination resistance. We demonstrated that all the cells with In2S3 IBL exhibited decelerated OCVD compared to the control cell without IBL. It reﬂects that the In2S3 IBL improves the recombination resistance of the cells, which is consistent with the dark J-V characteristics.
EIS was used to examine the interfacial characteristics of the cells as shown in Fig. 4, where the cell with optimized In2S3 IBL (i.e., 2In2S3) was compared with the control cell without IBL. Nyquist curves obtained from EIS measurements were ﬁtted to the equivalent circuit model shown in Fig. 4a. Depicted electrical elements Rs (series resistance), Rr1 (resistance at the counter electrode/electrolyte interface), Rr2 (resistance at the TiO2/QDs/electrolyte interface) and chemical phase elements Cm1 and Cm2, were collectively employed for modeling and hence describing the interfacial properties like recombination resistance and capacitance of QDSSCs. Fig. 4b shows the chemical capacitance of the cells (Cm) as a function of VF (Fermi voltage), where the chemical capacitance increased by adoption of 2In2S3 IBL compared to the control cell without IBL due to reduction in the direct contact area between TiO2 and electrolyte under the presence of In2S3 IBL. Such an increase in the chemical capacitance of the cells, which originates from the variation of number of electrons in the conduction band of TiO2, makes it unfair to compare its recombination resistance against the control cell
Fig. 4. (a) Equivalent circuit diagram used for ﬁtting the EIS data of PbS-QDSSCs with and without optimal In2S3 IBL. (b) Chemical capacitance as a function of VF, (c) recombination resistance and (d) electron life time as a function of VECB.
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Fig. 5. (a) Normalized current as a function of illumination time for PbS-QDSSC with and without In2S3 IBLs and (b) J-V characteristics of PbS-QDSSC with 2In2S3 IBL before and after annealing.
without In2S3 IBL. A common equivalent conduction band (VECB) condition is generally implemented to solve this disparity . For this reason, the voltage scale was shifted until the chemical capacitances of both cells overlapped each other (Fig. S4). Fig. 4c shows that the recombination resistance (Rr) of the cell increased due to In2S3 IBL incorporation, which conﬁrms the OCVD measurement inference. Since electron life time t (t ¼ CmRr) is directly related to the recombination resistance of a cell at a certain chemical capacitance (Cm), we also evolved that the electron life time of the cell with 2In2S3 IBL was superior to that of the control cell without In2S3 IBL as shown in Fig. 4d. Normalized current behavior is frequently studied to explore the stability of a QDSSC. Normalized current, a ratio between instantaneous value (It) and initial value (Iin) of photocurrent under illumination, is plotted as a function of time to examine the effect of In2S3 IBL on stability (Fig. 5a). The rate of photocorrosion of QDs is ﬁrmly dependent on photon ﬂux and energy of photons exposed to the QDs . In view of the fact that the absorption of high energy photons by In2S3 IBL with higher band gap energy than PbS allows relatively lower energy photons to transmit to PbS-QDs , the photocurrent stability was improved with increasing thickness of In2S3 IBL. On the other hand, annealing generally enhances the photovoltaic performance of QDSSCs with SILAR deposited QDs due to the improved crystallinity of QDs [45,46]. Hence, the best performing cell with 2In2S3 IBL was annealed at 250 C for 30 min in a conventional box furnace after QDs sensitization, which resulted an improvement in the PCE from 2.25 to 2.7% as shown in Fig. 5b. Consequently, the strategic improvement in PCE of PbS-QDSSCs can be summarized as follows: 2In2 S3 IBL
h ¼ 1:91%! 2:25% ! 2:70%
4. Conclusion In summary, we demonstrated signiﬁcant improvement in the photo-conversion efﬁciency of PbS-QDSSCs by introducing SILARdeposited In2S3 interfacial recombination barrier layer on mesoporous TiO2 anode, which is further improved by annealing the PbS-QDSSC with optimal In2S3 interfacial recombination barrier layer. Primarily, In2S3 interfacial recombination barrier layer hinders charge carrier recombination through the back-transfer of electrons in TiO2 anode to PbS and/or electrolyte, which is directly conﬁrmed by dark J-V, OCVD and EIS measurements. Moreover, the In2S3 interfacial recombination barrier layer improved the photocurrent stability of PbS-QDSSCs due to alleviation of high energy
photon ﬂux reaching PbS-QDs. However, an increase in In2S3 thickness beyond 2 SILAR cycles sloped down the photo-conversion efﬁciency of PbS-QDSSCs because In2S3 interfacial recombination barrier layer reduced the electron ﬂow from PbS-QDs to TiO2. Acknowledgments This work was supported by a New & Renewable Energy (No. 20123010010160) and Human Resources Development program (No. 20154030200680) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the South Korean government Ministry of Trade, Industry and Energy. This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2013R1A1A1008762). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2015.08.237. References  H. Choi, C. Nahm, J. Kim, C. Kim, S. Kang, T. Hwang, B. Park, Review paper: toward highly efﬁcient quantum-dot- and dye-sensitized solar cells, Curr. Appl. Phys. 13 (2013) S2eS13.  J. Luo, H. Wei, Q. Huang, X. Hu, H. Zhao, R. Yu, D. Li, Y. Luo, Q. Meng, Highly efﬁcient core-shell CuInS2-Mn doped CdS quantum dot sensitized solar cells, Chem. Commun. 49 (2013) 3881e3883.  P.K. Santra, P.V. Kamat, Mn-doped quantum dot sensitized solar cells: a strategy to boost efﬁciency over 5%, J. Am. Chem. Soc. 134 (2012) 2508e2511.  S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, F.E. CurchodBasile, N. Ashari€tzel, DyeAstani, I. Tavernelli, U. Rothlisberger, K. NazeeruddinMd, M. Gra sensitized solar cells with 13% efﬁciency achieved through the molecular engineering of porphyrin sensitizers, Nat. Chem. 6 (2014) 242e247.  J.W. Lee, D.Y. Son, T.K. Ahn, H.W. Shin, I.Y. Kim, S.J. Hwang, M.J. Ko, S. Sul, H. Han, N.G. Park, Quantum-dot-sensitized solar cell with unprecedentedly high photocurrent, Sci. Rep. 3 (2013) 1050. nez, I. Concina, A. Vomiero, I.n. Mora-Sero , Panchromatic  A. Braga, S. Gime sensitized solar cells based on metal sulﬁde quantum dots grown directly on nanostructured TiO2 electrodes, J. Phys. Chem. Lett. 2 (2011) 454e460.  E.H. Sargent, Colloidal quantum dot solar cells, Nat. Photonics 6 (2012) 133e135.  S.A. McDonald, G. Konstantatos, S. Zhang, P.W. Cyr, E.J. Klem, L. Levina, E.H. Sargent, Solution-processed PbS quantum dot infrared photodetectors and photovoltaics, Nat. Mater. 4 (2005) 138e142.  E.H. Sargent, Solar cells, photodetectors, and optical sources from infrared colloidal quantum dots, Adv. Mater. 20 (2008) 3958e3964.  X.-Y. Yu, B.-X. Lei, D.-B. Kuang, C.-Y. Su, Highly efﬁcient CdTe/CdS quantum dot sensitized solar cells fabricated by a one-step linker assisted chemical bath deposition, Chem. Sci. 2 (2011) 1396. lez-Pedro, X. Xu, I. Mora-Sero , J. Bisquert, Modeling high-efﬁciency  V. Gonza quantum dot sensitized solar cells, ACS Nano 4 (2010) 5783e5790.  L.H. Lai, L. Protesescu, M.V. Kovalenko, M.A. Loi, Sensitized solar cells with
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